1. Field of the Invention
The invention relates to insulation for covering high voltage components having irregular surfaces, and in particular concerns an improved filler material for a high voltage coil having Roebeled coil strands on its outer surface, wherein epoxy resin impregnated felt materials used to cover and insulate the Roebeled strands are wrapped in a low resistance conductive tape to reduce potential gradients across the filler material, thereby reducing the occurrence of partial discharges in the filler voids.
2. Prior Art
In order to minimize the losses which would occur in a multi-stranded coils as the result of eddy currents, the top and bottom strands of the coil are intertwined in a braid-like form, known as Roebeled strands. Roebeled strands form irregular surfaces on the top and bottom of the un-insulated coil.
Roebel fillers are insulating materials used for filling and smoothing the irregular surfaces formed by Roebeled strands. Roebel fillers heretofore known comprise resin impregnated felt materials or mica splittings which are bonded to the uneven coil surfaces by a process in which the impregnated materials are simultaneously heated and compressed, causing thermosetting resins to liquefy and fill voids of the irregular coil surfaces, and then to harden. The formed and bonded coil at this stage of manufacture is referred to as a "bakelized coil." Following completion of the bonding stage, a mica tape ground wall is formed around the consolidated coil and vacuum-impregnated with an epoxy resin. The coil is then pressed and baked to a final cure of the insulating structure.
Because the Roebel fillers are processed and cured to the coil surfaces under atmospheric conditions, air bubbles can be entrained in the fillers, forming voids in the hardened epoxy. Voids within the insulation system of high voltage stator coils can be a source of electrical discharges either during electrical testing or during operation of the coil in an electric machine. Electrical discharges or corona activity within a coil's insulation system can be detrimental to the integrity of the insulation and lead to early failure of the coil. In air cooled coils, partial discharges within the high voltage coil can lead to early failure of the complete stator winding.
Following the manufacture of the high voltage coils, each coil is subject to a series of electrical tests. One test that all coils must pass is the measurement of power factor tip-up. The power factor tip-up test results indicate how well consolidated the coil is, and the relative void content of the coil. A high voltage is applied to the coil under test, and the power factor is measured using a power factor bridge. Any internal discharges that occur in the coils cause an increase in the coils' power factor tip-up. The most likely source of partial discharges is unimpregnable, closed voids that are typically trapped bubbles in the epoxy resin associated with the Roebel filler material. Since the coil is bakelized at normal ambient pressure (1 atm.), the hardened epoxy resin traps some air. If trapped air in the cured epoxy resin is in the electric field path upon application of the power factor test voltage, then partial discharge activity can occur and cause a high power factor tip-up. The magnitude of power factor tip-up is a function of the level of applied voltage, the size of the void, and its position in the Roebel filler material.
The most effective way to eliminate electrical discharges in the insulating structure of a high voltage stator coil, and reduce power factor tip-up, is to produce an insulating structure which is void free. Since most voids are formed within the Roebel filler material by trapped air bubbles, one solution would be to process the filler coil in a vacuum. Unfortunately, this approach is cost prohibitive due to the expense associated with vacuum/heat impregnation. A more practical approach would be to prevent the voids within the Roebel filler material from discharging under the application of voltage.
U.S. Pat. No. 5,175,396, discloses a method for reducing partial discharges in filler voids which introduces a semi-conductive layer between the Roebel filler and the outer ground wall layer. The semi-conductive layer reduces the electrical stresses across the filler material to reduce the incidence of partial discharges in the filler voids.
U.S. Pat. No. 5,633,396, discloses a method for producing an electrically conductive felt impregnated an with epoxy resin, for insulating high voltage coils. The electrical pathways in the semiconductive felt reduce electrical stresses across the filler voids, thereby reducing the occurrence of electrical discharges in the filler materials.
While each of the above-mentioned methods has proven to reduce electrical discharges in the voids of filler materials used to insulate high voltage components, the manufacturing time and associated costs attributed to each process remain unfavorable. What is needed is an improved filler material for a high voltage coil having irregular surfaces, which is inexpensive to produce and effective in reducing the number of electrical discharges in the voids of the filler material under the application of high voltage.